In the original application of applicant's U.S. Pat. No. 2,597,046 there is an embodiment disclosed in which liquid metal is poured into an atomizer rotating around a vertical axis to project the atomized particles against the innermost diameter of a thoroid which is like a long helical spring bent around to form a circle, with the two ends joined together to look like a doughnut. The convolutions of said thoroid touch each other at its horizontal plan of symmetry (which is the plane of projection of the atomized particles), and form there a cylindrical surface.
The projected particles impinge against said cylindrical target and most of them solidify on contact to form a ring. Since said thoroid is slowly rotating around its circular axis and the deposited ring follows that surface, said ring is progressing downward to form a tube.
Said embodiment has become subject of a divisional application Ser. No. 286,076 which applicant has abandoned after the first action because of operating difficulties that were discovered. The atomized particles did not lose much heat before reaching their target and moreover, the contact with the heat-absorbing thoroid was very short. Consequently the production rate was too slow for an industrial unit. Besides, there were frequent interruptions when a part of an already formed tube fell down because it was not sufficiently solidified.
Not until recently has applicant been able to find how that original embodiment can be made to work. By combining the action of a few elements, he obtained rather unexpected results that not only assure industrial production of plates and other flat products but permit to do so on a scale surpassing by far existing industrial processes. The method consists in:
1. Reshaping the atomizer to admit 3 to 4 times the quantity of gas needed for atomizing, while creating sufficient turbulence for the excess gas to absorb heat from the atomized particles so that at least half of them reach their target in a solid, though plastic, state.
2. Using a pneumatic motor to rotate the atomizer brought the speed to over 6000 r.p.m. while taking advantage of the cooling effect of the expanding gas to insure dependability of operation in close proximity of molten steel at 1600.degree. C.
3. The centrifugal force acting on the atomized particles at such speed aided by the above mentioned current of cooling gas from the atomizer has in turn permitted to place the target a considerable distance farther from the atomizer, as much as 75 to 100 ft. diameter which gives the atomized metal a colossal area of contact with the target to absorb the remaining melting heat in the metal and even to lower its temperature somewhat.
4. By enclosing the umbrella of projected atomized particles between heat-absorbing roof and floor, to form a gas-tight chamber for the non-oxydizing gas, cooling of said particles by radiation is increased. On top of it, the atomizer itself is made to oscillate vertically by several inches or even more (always within the space between said roof and floor of the gas-tight chamber). This step also increases several times the area of the target.
5. Furthermore, applicant is providing inside and out, heat absorbing fences surrounding the product, i.e. the huge tubular body, and thus, as an extra effect, has created means for an almost total recuperation of the heat of the molten metal by means which are accessible, easy to use and to maintain.
By fairly accurate calculation such continuous spray caster built to produce a 75 ft. diameter tube has ample capacity to produce 500 tons per hour which is about 3,000,000 tons a year. This is enough for many steel works to constitute the only caster for the general mix of flat products, from plates and tubes, through autobody and other sheets down to tinplates. The cost of such unit is high, but is considerably less than two or three conventional units that could jointly achieve the same tonnage.
Such products possess a homogeneous, small-grained, almost non-porous metal structure which is characteristic of spray casting and are completely free of segregations that are unavoidable with conventional continuous casting.
When passing molten steel through an atomizer, certain alloying elements such as Ni, Mo, Va may be added to alloy with the steel. Measured quantities of such metals in powder form are injected together with the steel. If the percentage of the alloying elements is high, the steel should be sufficiently superheated.
For steel qualities requiring a steel of less exacting analysis, such as rebars, the subject process permits to eliminate the steel-making operation altogether, resulting in substantial reduction of costs.
Pig iron from the blast furnace, after removal of sulphur and silicon, is passed through the atomizer together with measured quantities of iron-oxide which may be mill scale or refined iron ore which is reduced to iron after the particles have reached their target, by combining with the carbon of which the pig iron may have 3 to 5%. The quantity of the iron oxide which is added must be calculated to leave some excess carbon to produce steel with the carbon content required.